Changes in cell morphology of Listeria monocytogenes and Shewanella putrefaciens resulting from the action of protamine

Protamine, which is an antibacterial basic peptide, was shown to alter the cell morphology of Listeria monocytogenes and Shewanella putrefaciens. Atomic force microscopy revealed that protamine smoothed the surface of cells, formed holes in the cell envelope, and caused fusion of S. putrefaciens cells. Immunoelectron microscopy of protamine-treated cells of both L. monocytogenes and S. putrefaciens showed great damage to the cell wall and condensation of the cytoplasm. Respiration of the cells was decreased due to treatment with sublethal concentrations of protamine, probably due to leakage or loss of cell envelope potential. It was concluded that protamine disrupted the outer surface structure and condensed the cytoplasm of sensitive cells and, in sublethal concentrations, altered membrane structures, thereby eliminating respiration.